Pass the regrtest worker configuration in a temporary file so a large --matchfile does not exceed the operating system's command-line size limit.

Keep the file for the worker's lifetime using the existing cleanup stack, close it before launching the worker so Windows can reopen it, and keep standard input available to tests. Add a regression that selects one test using a match file larger than Linux's per-argument limit.

For custom runners, create the configuration in the parent's test directory and pass its path relative to libregrtest. The worker resolves it against its own module location, allowing WASI runners to map the source tree to a different filesystem root.

Fixes #139207.

Validation:

• The new regression reproduces OSError: [Errno 7] Argument list too long before the fix.
• Full regrtest suite on a CPython debug build: 140 tests, 3 skipped.
• A custom runner using a separate source path and working directory successfully loads its configuration and runs a test.
• Mypy 2.3.1 under Python 3.15: no issues in 21 source files.
• Ruff, NEWS lint, and patchcheck.